Fix data loss for PUB/SUB and unidirectional transports (LIBZMQ-268)

With the introduction of subscription forwarding, the first message sent
on a PUB socket using a unidirectional transport (e.g. PGM) is always
lost due to the "subscribe to all" being done asynchronously.

This patch fixes the problem and also refactors the code to have a single
point where the "subscribe to all" is performed.

Signed-off-by: Martin Lucina <martin@lucina.net>

src/pull.hpp

@@ -45,7 +45,7 @@ namespace zmq
     protected:
 
         //  Overloads of functions from socket_base_t.
-        void xattach_pipe (zmq::pipe_t *pipe_);
+        void xattach_pipe (zmq::pipe_t *pipe_, bool icanhasall_);
         int xrecv (zmq::msg_t *msg_, int flags_);
         bool xhas_in ();
         void xread_activated (zmq::pipe_t *pipe_);